Car manufacturers have traditionally used glass as the material for car windows. However, recently it has become desirable to substitute glass windows with windows made of a plastic or polymer resin such as polycarbonate (PC). Polymeric windows are often advantageous compared to glass windows because of their low weight, high strength and ease of shaping.
One potential limitation to the use of polymeric windows in cars is that infrared radiation passes through the windows unfiltered. This results in a heavy thermal load on air conditioning and also affects passenger comfort. In general, it is believed that the automotive industry will not switch from glass to polymeric windows unless IR filtering equal to automotive glass is met.
One known approach to providing IR filtering for glass is to provide an IR reflecting metal layer, such as Ag or Al. However, these metals are chemically unstable and will degrade in air, forming opaque metal oxides and sulfides unless protected. Thus, IR reflecting layers are typically used only in the middle of double pane glass structures where they are protected from exposure to air or water.
A second known IR filtering approach used on glass is the application of an IR reflecting oxide, such as indium tin oxide (ITO) to the glass substrate. ITO is electrically conductive and reflects well in the IR (i.e. wavelengths above 1500 nm). However, ITO does not reflect well in the near IR (i.e. wavelengths between 800 and 1500 nm). Filtering in the near IR is important for vehicle applications to prevent the vehicle cabin from overheating. The electromagnetic spectrum containing the IR, near IR, visible and UV regions is shown in FIG. 1.
A third approach that has been demonstrated on glass windows is to sandwich an Ag or Al infrared reflecting layer between two titanium dioxide (TiO.sub.2) or zinc oxide (ZnO) layers. This approach, however, is unsuitable for polymeric substrates for several reasons. For example, the TiO.sub.2 containing coating acts as only a partial absorber of UV radiation. Because TiO.sub.2 provides an insufficient amount of UV absorption, polymeric windows coated with TiO.sub.2 would eventually turn a shade of yellow. Furthermore, TiO.sub.2 is photocatalytic and will degrade PC. Therefore, TiO.sub.2 coated polymers are generally unsuitable as a car window material.
ZnO provides a greater amount of UV absorption than TiO.sub.2 and provides UV absorption to higher wavelengths. Thus, ZnO would provide enough UV protection to prevent polymers from turning yellow when exposed to sunlight. However, a ZnO layer fabricated by prior art methods is known to dissolve in water. Thus, a prior art ZnO layer typically has an unacceptable water soak stability, as exhibited by increased haze and decreased optical density after being soaked in water for an extended period of time.